Currently, a white organic light-emitting diode (OLED) includes various structures, such as a laminated structure consisting of a plurality of elements, a color conversion structure, a structure including a single light-emitting layer, and a structure consisting of several light-emitting layers. The structure consisting of several light-emitting layers has been widely used in the white OLED due to its simple manufacturing process and excellent color stability.
As shown in FIG. 1, a conventional OLED with the structure of several light-emitting layers includes a cathode, an anode, and several organic light-emitting layers arranged between the cathode and the anode. As its working principle, when a forward bias voltage derived from a direct current (DC) is applied to an element, electrons and holes are driven by such an additional voltage into the organic light-emitting layers via the cathode and the anode, respectively. When organic molecules with an unstable excitation state are changed from an excitation state to a ground state, a luminescent phenomenon occurs. After the organic molecules are excited by external energy, the organic molecules are in a singlet state when the excitation-state electrons and ground-state electrons have paired electron spin, and the light generated thereby is just the so-called fluorescence. In contrast, when the excitation-state electrons and the ground-state electrons have parallel electron spin, the organic molecules are in a triplet state, and the light generated thereby is just the so-called phosphorescence. The phosphorescence OLED is four times the light-emitting efficiency of the fluorescence OLED, and thus has been widely used nowadays.
Phosphorescent materials capable of being excited to emit red light, yellow light and green light have excellent characteristics, and various materials are available for selection, so they have recently been putted into mass production. However, a phosphorescent material capable of being excited to emit blue light is not commercially available. As a result, in the white OLED with the structure consisting of several light-emitting layers, a fluorescent material used to emit the blue light, has a low blue light-emitting efficiency, which result in the low light-emitting efficiency of the white OLED. Of course, the fluorescent material with a low light-emitting efficiency may also be a material capable of being excited to emit the light in other colors.